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TitleTectonic evolution of the Aggeneys-Gamsberg Ore District, South Africa, and implications for the geodynamic setting of the Namaqua Sector
AuthorCawood, T KORCID logo; Rozendaal, A; Buick, I; Frei, D
SourceOre Geology Reviews vol. 157, 105440, 2023 p. 1-34, Open Access logo Open Access
Alt SeriesNatural Resources Canada, Contribution Series 20230135
Mediapaper; digital; on-line
File formatpdf
AreaNamaqua Sector; South Africa
Lat/Long WENS 16.9167 19.7500 -28.8333 -31.6667
SubjectsScience and Technology; general geology; tectonics; deformation; geodynamics; sulphides; Aggeneys-Gamsberg Ore District
Illustrationslocation maps; charts; diagrams; photographs; tables
Released2023 04 20
AbstractUnderstanding the deformation history of the structurally complex Aggeneys-Gamsberg Ore District (A-GOD), in the Namaqua Sector of the Mesoproterozoic Namaqua-Natal Metamorphic Province of South Africa, is key to discovering additional metasediment-hosted base metal sulphide mineralization. Although several studies have examined the structure of the main ore deposits and outcrops in the district, and others have investigated the timing of sedimentation, magmatism, and metamorphism, no previous studies have focused on integrating these two datasets. We therefore combine structural observations and the resulting relative timeline of events with new and existing zircon U-Pb geochronology, to yield a temporally constrained tectonic history for the district. The regionally recognized Paleoproterozoic D1 deformation is not recorded in the A-GOD, which preserves only Mesoproterozoic rocks. The largely stratiform base metal deposits occur near the top of a sequence of metamorphosed siliciclastic sediments (Wortel and Hotson Formations), deposited after 1261 ± 5 Ma (youngest reliable detrital zircons). These metasediments are tectonically interleaved with the intrusive ~1190 Ma Little Namaqualand Suite (Aroams Gneiss) and younger ~1175-1145 Ma metagranitoids (Achab Gneiss, Hoogoor Gneiss, aplite sills). Magmatism occurred early during D2a deformation, which created kilometre-scale isoclinal folds and thrusts during S/SW-directed contraction, as recorded at the Swartberg and Broken Hill-Deeps deposits. Associated M2a metamorphism is dated at ~1200 Ma (metamorphic zircon rims), but this was not the peak metamorphic episode as previously thought. The sequence was then exhumed, and the unconformably overlying clastics and bimodal volcanics of the Koeris Formation were deposited in local transtensional pull-apart basins at ~1130 Ma. Thrust stacking in the Aggeneys Mountains may have occurred during subsequent D2c contraction, associated with ~1075 Ma M2c metamorphism, but evidence remains cryptic. The entire sequence is affected by kilometre-scale sheath folds that close downwards to the ~ENE (as at Gamsberg and the Quarry Fold Nappe), associated with ~ENE-trending open, upright folds. These developed during D3a ~ENE-directed extensional to constrictional shearing at peak M3 amphibolite-facies conditions, which re-used the gently dipping D2 regional fabric and overprinted it with new ~E-W trending D3 linear features. Abundant metamorphic zircon dates suggest an age of ~1025 Ma for D3a and M3. Ongoing extension at retrograde conditions reactivated earlier structures, such as the Swartberg Thrust, as D3b low-angle, ductile-brittle detachment faults. Finally, dextral transform motion created local D4 monoclines and dextral strike-slip faults, possibly associated with ~1000 Ma M4 metamorphic zircon growth and emplacement of a regional pegmatite swarm. The A-GOD has traditionally been fitted into a regional tectonic model involving accretion of distinct terranes onto the Kaapvaal Craton, during the polyphase ~1.2-1.0 Ga Namaquan Orogeny. In contrast, recent studies propose that the entire Namaqua Sector instead occupied a back-arc setting between ~1.35 and <1.1 Ga. Our results support a hybrid model, involving accretion and translation of previously extended crustal fragments against the craton, with initially the entire Namaqua Sector - but later only the western portion - located in a hot back-arc setting influenced by subducting slab dynamics.
Summary(Plain Language Summary, not published)
The Aggeneys-Gamsberg Ore District in South Africa contains several ore deposits, with copper, lead, zinc, and silver. These deposits have been buried, intruded by magma, deformed, and metamorphosed at high temperatures, possibly several times. This makes the district very complex, so it is difficult to find new deposits. We wanted to understand the history of the district, to make finding new ore deposits easier. To do this, we analyzed zircon crystals, which can tell you when the zircon was formed, and whether it grew in a magma or during metamorphism. We also studied the shapes and types of rocks in the district, and combined our results with information from previous studies. We found that the ore deposits and the sedimentary rocks they occur in were deposited around 1261 million years ago. They were then buried, deformed, metamorphosed, and intruded by magmas at approximately 1190-1145 million years ago, when a large mountain belt was created. The area was then eroded, and new volcanic and sedimentary rocks were laid down 1130 million years ago, on top of the old ore deposits. The entire package was then buried again and metamorphosed 1075 million years ago, forming a second mountain belt. At 1025 million years ago this mountain belt collapsed, and the rocks were stretched and thinned. Finally, 1000 million years ago, the district was cut by brittle faults.

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